Normally tacky pressure-sensitive adhesive (hereinafter identified by the abbreviation "psa") compositions suitable, for example, for use in adhesive tapes must have a requisite fourfold balance of adhesion, cohesion, stretchiness and elasticity. Psa coated tapes have been produced for at least a half a century. The expectation level of the performance of early psa coated tapes was, to say the least, not great. Early psa tapes were expected to at least temporarily adhere to the surface upon which they were adhered and certain minor problems such as adhesive failure, discoloration, cohesive failure, etc. were tolerated. As psas became more sophisticated, mainly because of research in this area, the expectation level of the performance of the psa on coated tapes reached an extremely high level.
Some psa compositions desirably have transparency and resistance to sunlight aging even on exposure to severe weather conditions. With environmental considerations being more important, solvent-free processability is also a desired but often elusive feature.
Early psa compositions consisted of natural crude rubber tackified by admixture with esterified wood rosin. This was later improved by the introduction of synthetic acrylic ester elastomers which were inherently tacky and possessed the elasticity and compliance required for the fourfold balance of properties of a true psa. As the need arose, improvements were made in the basic acrylic ester psa to meet the needs in the marketplace. Transparency and resistance to oxidation inherent in acrylic ester psas made them outstanding candidates for the most demanding psa tape applications.
Many block copolymers have psa properties and have cohesive strength and hot melt processability, but they do not have the oxidative resistance or the optical clarity of the acrylic ester adhesives. Various references teach block copolymer psa compositions, but not how to improve the latter properties. Instead, Harlan (U.S. Pat. No. 3,239,478) teaches how "oil-tolerant" they can be, Korpman (U.S. Pat. No. 3,625,752) and Downey (U.S. Pat. Nos. 3,880,953 and 3,954,692) teach how to improve adhesion through use of specifically formulated tackifiers, and Freeman (U.S. Pat. No. 4,102,835) and Korpman (U.S. Pat. No. 4,136,071) use combinations of ABA and AB copolymers to extend the range of performance.
Psa systems which by their nature are adhesives which have an extremely delicate balance of properties known in the trade as the "fourfold" balance of adhesion, cohesion, stretchiness and elasticity are described in U.S. Pat. No. 2,884,176. The desire to maintain this balance of properties makes it extremely difficult to improve internal strength, i.e., the cohesiveness without also upsetting the other properties and destroying the overall pressure-sensitive nature of the adhesive system.
The prior art relating to "graft" copolymers does not deal with psa systems. The prior art related to "graft" copolymers is directed to modifying systems which are not pressure-sensitive and for purposes diametrically opposed to the teaching of the present application. The patents of Behrens (U.S. Pat. No. 3,004,958), Gregorian (U.S. Pat. No. 3,135,717), Milkovich (U.S. Pat. Nos. 3,786,116; 3,832,423; 3,862,267) teach how to graft side chains of polystyrene or acrylate esters onto rigid or semi-rigid backbones of polyvinyl chloride or methacrylate polymers to provide flexibility and temperature and impact resistance. Harlan (U.S. Pat. No.4,007,311) shows that grafting methyl methacrylate to a styreneisoprene-styrene block copolymer enhances adhesion without regard for elasticity or cohesiveness. In Ambrose (U.S. Pat. No. 4,075,186), a butadiene side chain is grafted to an acrylate polymer backbone to produce a molding material which has improved electrical properties and impact resistance but which is tack-free.
An acrylic psa having versatile processing capabilities and improved shear strength, to applicants' knowledge, is not known. Applicants herein teach the preparation of such an adhesive without sacrificing the outstanding optical clarity and resistance to oxidative and photochemical forces of the acrylic ester copolymer backbone.